DESCRIPTION

These macros define and operate on five types of data structures:
singly-linked lists, simple queues, lists, tail queues, and circular
queues. All five structures support the following functionality:
1. Insertion of a new entry at the head of the list.
2. Insertion of a new entry after any element in the list.
3. Removal of an entry from the head of the list.
4. Forward traversal through the list.
Singly-linked lists are the simplest of the five data structures and sup-
port only the above functionality. Singly-linked lists are ideal for ap-
plications with large datasets and few or no removals, or for implement-
ing a LIFO queue.
Simple queues add the following functionality:
1. Entries can be added at the end of a list.
However:
1. All list insertions must specify the head of the list.
2. Each head entry requires two pointers rather than one.
3. Code size is about 15% greater and operations run about 20%
slower than singly-linked lists.
Simple queues are ideal for applications with large datasets and few or
no removals, or for implementing a FIFO queue.
All doubly linked types of data structures (lists, tail queues, and cir-
cle queues) additionally allow:
1. Insertion of a new entry before any element in the list.
2. Removal of any entry in the list.
However:
1. Each element requires two pointers rather than one.
2. Code size and execution time of operations (except for remo-
val) is about twice that of the singly-linked data-structures.
Lists are the simplest of the doubly linked data structures and support
only the above functionality over singly-linked lists.
Tail queues add the following functionality:
1. Entries can be added at the end of a list.
2. They may be traversed backwards, at a cost.
However:
1. All list insertions and removals must specify the head of the
list.
2. Each head entry requires two pointers rather than one.
3. Code size is about 15% greater and operations run about 20%
slower than singly-linked lists.
Circular queues add the following functionality:
1. Entries can be added at the end of a list.
2. They may be traversed backwards, from tail to head.
However:
1. All list insertions and removals must specify the head of the
list.
2. Each head entry requires two pointers rather than one.
3. The termination condition for traversal is more complex.
4. Code size is about 40% greater and operations run about 45%
slower than lists.
In the macro definitions, TYPE is the name tag of a user defined struc-
ture that must contain a field of type SLIST_ENTRY, LIST_ENTRY,
SIMPLEQ_ENTRY, TAILQ_ENTRY, or CIRCLEQ_ENTRY, named NAME. The argument
HEADNAME is the name tag of a user defined structure that must be de-
clared using the macros SLIST_HEAD(), LIST_HEAD(), SIMPLEQ_HEAD(),
TAILQ_HEAD(), or CIRCLEQ_HEAD(). See the examples below for further ex-
planation of how these macros are used.
SINGLY-LINKED LISTS
A singly-linked list is headed by a structure defined by the SLIST_HEAD()
macro. This structure contains a single pointer to the first element on
the list. The elements are singly linked for minimum space and pointer
manipulation overhead at the expense of O(n) removal for arbitrary ele-
ments. New elements can be added to the list after an existing element or
at the head of the list. A SLIST_HEAD structure is declared as follows:
SLIST_HEAD(HEADNAME, TYPE) head;
where HEADNAME is the name of the structure to be defined, and struct
TYPE is the type of the elements to be linked into the list. A pointer to
the head of the list can later be declared as:
struct HEADNAME *headp;
(The names head and headp are user selectable.)
The HEADNAME facility is often not used, leading to the following bizarre
code:
SLIST_HEAD(, TYPE) head, *headp;
The SLIST_ENTRY() macro declares a structure that connects the elements
in the list.
The SLIST_INIT() macro initializes the list referenced by head.
The list can also be initialized statically by using the
SLIST_HEAD_INITIALIZER() macro like this:
SLIST_HEAD(HEADNAME, TYPE) head = SLIST_HEAD_INITIALIZER(head);
The SLIST_INSERT_HEAD() macro inserts the new element elm at the head of
the list.
The SLIST_INSERT_AFTER() macro inserts the new element elm after the ele-
ment listelm.
The SLIST_REMOVE_HEAD() macro removes the first element of the list
pointed by head.
The SLIST_REMOVE_NEXT() macro removes the list element immediately fol-
lowing elm.
The SLIST_REMOVE() macro removes the element elm of the list pointed by
head.
The SLIST_FIRST() and SLIST_NEXT() macros can be used to traverse the
list:
for (np = SLIST_FIRST(&head); np != NULL; np = SLIST_NEXT(np, NAME))
Or, for simplicity, one can use the SLIST_FOREACH() macro:
SLIST_FOREACH(np, head, NAME)
The SLIST_FOREACH_PREVPTR() macro is similar to SLIST_FOREACH() except
that it stores a pointer to the previous element in VARNAMEP. This pro-
vides access to the previous element while traversing the list, as one
would have with a doubly-linked list.
The SLIST_EMPTY() macro should be used to check whether a simple list is
empty.

LISTS

A list is headed by a structure defined by the LIST_HEAD() macro. This
structure contains a single pointer to the first element on the list. The
elements are doubly linked so that an arbitrary element can be removed
without traversing the list. New elements can be added to the list after
an existing element, before an existing element, or at the head of the
list. A LIST_HEAD structure is declared as follows:
LIST_HEAD(HEADNAME, TYPE) head;
where HEADNAME is the name of the structure to be defined, and struct
TYPE is the type of the elements to be linked into the list. A pointer to
the head of the list can later be declared as:
struct HEADNAME *headp;
(The names head and headp are user selectable.)
The HEADNAME facility is often not used, leading to the following bizarre
code:
LIST_HEAD(, TYPE) head, *headp;
The LIST_ENTRY() macro declares a structure that connects the elements in
the list.
The LIST_INIT() macro initializes the list referenced by head.
The list can also be initialized statically by using the
LIST_HEAD_INITIALIZER() macro like this:
LIST_HEAD(HEADNAME, TYPE) head = LIST_HEAD_INITIALIZER(head);
The LIST_INSERT_HEAD() macro inserts the new element elm at the head of
the list.
The LIST_INSERT_AFTER() macro inserts the new element elm after the ele-
ment listelm.
The LIST_INSERT_BEFORE() macro inserts the new element elm before the
element listelm.
The LIST_REMOVE() macro removes the element elm from the list.
The LIST_REPLACE() macro replaces the list element elm with the new ele-
ment elm2.
The LIST_FIRST() and LIST_NEXT() macros can be used to traverse the list:
for (np = LIST_FIRST(&head); np != NULL; np = LIST_NEXT(np, NAME))
Or, for simplicity, one can use the LIST_FOREACH() macro:
LIST_FOREACH(np, head, NAME)
The LIST_EMPTY() macro should be used to check whether a list is empty.

SIMPLE QUEUES

A simple queue is headed by a structure defined by the SIMPLEQ_HEAD()
macro. This structure contains a pair of pointers, one to the first ele-
ment in the simple queue and the other to the last element in the simple
queue. The elements are singly linked. New elements can be added to the
queue after an existing element, at the head of the queue or at the tail
of the queue. A SIMPLEQ_HEAD structure is declared as follows:
SIMPLEQ_HEAD(HEADNAME, TYPE) head;
where HEADNAME is the name of the structure to be defined, and struct
TYPE is the type of the elements to be linked into the queue. A pointer
to the head of the queue can later be declared as:
struct HEADNAME *headp;
(The names head and headp are user selectable.)
The SIMPLEQ_ENTRY() macro declares a structure that connects the elements
in the queue.
The SIMPLEQ_INIT() macro initializes the queue referenced by head.
The queue can also be initialized statically by using the
SIMPLEQ_HEAD_INITIALIZER() macro like this:
SIMPLEQ_HEAD(HEADNAME, TYPE) head = SIMPLEQ_HEAD_INITIALIZER(head);
The SIMPLEQ_INSERT_HEAD() macro inserts the new element elm at the head
of the queue.
The SIMPLEQ_INSERT_TAIL() macro inserts the new element elm at the end of
the queue.
The SIMPLEQ_INSERT_AFTER() macro inserts the new element elm after the
element listelm.
The SIMPLEQ_REMOVE_HEAD() macro removes the first element from the queue.
The SIMPLEQ_FIRST() and SIMPLEQ_NEXT() macros can be used to traverse the
queue. The SIMPLEQ_FOREACH() is used for queue traversal:
SIMPLEQ_FOREACH(np, head, NAME)
The SIMPLEQ_EMPTY() macro should be used to check whether a list is emp-
ty.

TAIL QUEUES

A tail queue is headed by a structure defined by the TAILQ_HEAD() macro.
This structure contains a pair of pointers, one to the first element in
the tail queue and the other to the last element in the tail queue. The
elements are doubly linked so that an arbitrary element can be removed
without traversing the tail queue. New elements can be added to the queue
after an existing element, before an existing element, at the head of the
queue, or at the end of the queue. A TAILQ_HEAD structure is declared as
follows:
TAILQ_HEAD(HEADNAME, TYPE) head;
where HEADNAME is the name of the structure to be defined, and struct
TYPE is the type of the elements to be linked into the tail queue. A
pointer to the head of the tail queue can later be declared as:
struct HEADNAME *headp;
(The names head and headp are user selectable.)
The TAILQ_ENTRY() macro declares a structure that connects the elements
in the tail queue.
The TAILQ_INIT() macro initializes the tail queue referenced by head.
The tail queue can also be initialized statically by using the
TAILQ_HEAD_INITIALIZER() macro.
The TAILQ_INSERT_HEAD() macro inserts the new element elm at the head of
the tail queue.
The TAILQ_INSERT_TAIL() macro inserts the new element elm at the end of
the tail queue.
The TAILQ_INSERT_AFTER() macro inserts the new element elm after the ele-
ment listelm.
The TAILQ_INSERT_BEFORE() macro inserts the new element elm before the
element listelm.
The TAILQ_REMOVE() macro removes the element elm from the tail queue.
TAILQ_FOREACH() and TAILQ_FOREACH_REVERSE() are used for traversing a
tail queue. TAILQ_FOREACH() starts at the first element and proceeds to-
wards the last. TAILQ_FOREACH_REVERSE() starts at the last element and
proceeds towards the first.
TAILQ_FOREACH(np, &head, NAME)
TAILQ_FOREACH_REVERSE(np, &head, HEADNAME, NAME)
The TAILQ_FIRST(), TAILQ_NEXT(), TAILQ_LAST() and TAILQ_PREV() macros can
be used to manually traverse a tail queue or an arbitrary part of one.
The TAILQ_EMPTY() macro should be used to check whether a tail queue is
empty.

CIRCULAR QUEUES

A circular queue is headed by a structure defined by the CIRCLEQ_HEAD()
macro. This structure contains a pair of pointers, one to the first ele-
ment in the circular queue and the other to the last element in the cir-
cular queue. The elements are doubly linked so that an arbitrary element
can be removed without traversing the queue. New elements can be added to
the queue after an existing element, before an existing element, at the
head of the queue, or at the end of the queue. A CIRCLEQ_HEAD structure
is declared as follows:
CIRCLEQ_HEAD(HEADNAME, TYPE) head;
where HEADNAME is the name of the structure to be defined, and struct
TYPE is the type of the elements to be linked into the circular queue. A
pointer to the head of the circular queue can later be declared as:
struct HEADNAME *headp;
(The names head and headp are user selectable.)
The CIRCLEQ_ENTRY() macro declares a structure that connects the elements
in the circular queue.
The CIRCLEQ_INIT() macro initializes the circular queue referenced by
head.
The circular queue can also be initialized statically by using the
CIRCLEQ_HEAD_INITIALIZER() macro.
The CIRCLEQ_INSERT_HEAD() macro inserts the new element elm at the head
of the circular queue.
The CIRCLEQ_INSERT_TAIL() macro inserts the new element elm at the end of
the circular queue.
The CIRCLEQ_INSERT_AFTER() macro inserts the new element elm after the
element listelm.
The CIRCLEQ_INSERT_BEFORE() macro inserts the new element elm before the
element listelm.
The CIRCLEQ_REMOVE() macro removes the element elm from the circular
queue.
The CIRCLEQ_FIRST(), CIRCLEQ_LAST(), CIRCLEQ_END(), CIRCLEQ_NEXT() and
CIRCLEQ_PREV() macros can be used to traverse a circular queue. The
CIRCLEQ_FOREACH() is used for circular queue forward traversal:
CIRCLEQ_FOREACH(np, head, NAME)
The CIRCLEQ_FOREACH_REVERSE() macro acts like CIRCLEQ_FOREACH() but
traverses the circular queue backwards.
The CIRCLEQ_EMPTY() macro should be used to check whether a circular
queue is empty.

NOTES

The SLIST_END(), LIST_END(), SIMPLEQ_END() and TAILQ_END() macros are
provided for symmetry with CIRCLEQ_END(). They expand to NULL and don't
serve any useful purpose.
Trying to free a list in the following way is a common error:
LIST_FOREACH(var, head, entry)
free(var);
free(head);
Since var is free'd, the FOREACH() macro refers to a pointer that may
have been reallocated already. Proper code needs a second variable.
for (var = LIST_FIRST(head); var != LIST_END(head); var = nxt) {
nxt = LIST_NEXT(var, entry);
free(var);
}
LIST_INIT(head); /* to put the list back in order */

HISTORY

The queue functions first appeared in 4.4BSD.
MirOS BSD #10-current December 13, 1993 11